In limited power systems (including, but not limited to, power systems on an aircraft), a closed loop control system is often implemented to control the output voltage of a generator to ensure proper operation of devices attached to the power system.
A typical closed loop control system for power generation is shown schematically in FIG. 1. The system has a generator 11 and a reference sensor 12, which senses the output 13 and provides a reference signal 14 fed back to the generator. The output is connected to a load 15. The reference signal has a finite frequency response to allow for control of the output. As the output changes, the reference sensor adjusts the generator to remain at a constant output.
A critical point may be reached in a closed loop system where the input impedance Zin of the load 15 is lower than the output impedance Zout of the generator 11. In this case, the system can revert from closed loop control to open loop control and possibly into sub-harmonic oscillation or modulation of the output voltage.
The onboard power generation system for a modern passenger aircraft must accommodate different types of loads. In the case of galleys, heaters, air conditioners, etc. the load is resistive, so that a decrease in the voltage input to the load causes a decrease in the load current. In contrast, inflight entertainment (IFE) systems and other electronic systems generally have a reactive load with a negative impedance or constant power characteristic, so that as the generator voltage decreases the load current increases. As the IFE system load increases, the impedance Zin decreases, and may become lower than the output dynamic impedance Zout of the generator. When this happens, the generator can go into a “hunting” mode in an attempt to regain closed loop control. This behavior typically presents itself as a voltage modulation about the mean of the voltage set point, characterized by a voltage modulation envelope.
The power generating system may be designed to tolerate a certain modulation envelope size (maximum voltage peak to valley). The modulation may be successfully limited as long as the load remains within the boundaries for closed loop control. Accordingly, it is desirable to implement a modulation monitor and control system to control the load, limit the modulation envelope, and thereby ensure that the system stays within operational boundaries.